Grid selection in the North Sea industrial trawl fishery for Norway pout: efficient size selection reduces bycatch

Published source details Kvalsvik K., Huse I., Misund O.A. & Gamst K. (2006) Grid selection in the North Sea industrial trawl fishery for Norway pout: efficient size selection reduces bycatch. Fisheries Research, 77, 248-263.
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This study is summarised as evidence for the following.

	Action	Category
	Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water) Action Link
	Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net Action Link

Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

A replicated study in 1997–1999 of a seabed area in the North Sea off Norway (Kvalsvik et al. 2006) found that using a different configuration of size-sorting escape grid (decreased bar spacing and bar thickness) in fish trawl nets increased the escape of larger individuals of unwanted haddock Melanogrammus aeglefinus in an industrial trawl fishery for Norway pout Trisopterus esmarkii. In the 1997 trials, the average percentage of non-target haddock that was sorted out by the grids (i.e. too large to pass through the grid into the codend) increased with decreasing bar spacing (19 mm: 77%, 22mm: 41%, 25 mm: 34%. However, the sorted-out haddock (all grids) were of larger lengths than those retained in the codends (data reported as cumulative length frequencies). In separate trials in 1998–1999, the length at which haddock had a 50% chance of not passing through the grid into the codend was smaller with a thinner 5 mm bar thickness of grid (18 cm) than either a 10 mm or 15 mm grid (both 19.4 cm). This was found to indicate that more larger individuals were able to escape from the grid with the bar spacing that had the highest flow of water (as determined separately in flume tank tests). Data were collected from three trials on two vessels in June 1997, May 1998 and September/October 1999 using trawl nets fitted with 1.4 × 1.9 grids and top escape opening. Trials in 1997 used grids of 19 mm, 22 mm and 25 mm bar spacing. The 22 mm grid only was used in 1998 and 1999 for grids with either 5 mm, 10 mm or 15 mm bar thickness. A small mesh cover over the escape opening collected the catch escaping via each grid.

(Summarised by: Leo Clarke)
Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net

A replicated study in 1997–1999 of a seabed area in the North Sea off Norway (Kvalsvik et al. 2006) found that fish trawl nets fitted with size-sorting escape grids of various configurations allowed unwanted haddock Melanogrammus aeglefinus and other non-target fish to escape capture in an industrial trawl fishery for Norway pout Trisopterus esmarkii, relative to the total catch. In 1997 trials, overall percentages of target and non-target fish sorted out by all grid configurations was 13–58%. For the only non-target species with data, haddock, an average of 44% escaped capture and they were significantly smaller than those retained in the codend (data reported as statistical results). In separate trials in 1998–1999 using different grid/net configurations, averages of 25–73% of unwanted haddock, and 65–100% of eight of eight other non-target species (saithe Pollachius virens, whiting Merlangius merlangus, Atlantic cod Gadus morhua, ling Molva molva, hake Merluccius merluccius, mackerel Scomber scombrus, herring Clupea harengus, tusk Brosme brosme) escaped capture. Data were collected from 35 trawl deployments with grids fitted in 1997 and 60 in 1998–1999. In 1997, different guiding panels and grid bar spacings were tested and in 1998–1999 combinations of grid bar thickness and mesh size of netting where grids were inserted were tested (see paper for full specifications). Escaping fish were collected in a cover attached over the escape opening. Cover and codend fish catches were sampled.

(Summarised by: Leo Clarke)